1. Technical Field
The present invention relates to a method of executing water jet peening and, in particular, to a method of executing water jet peening suitable for structural members of a nuclear power plant.
2. Background Art
In a nuclear power plant, when there exists tensile residual stress on a surface of a weld of a structural member of a reactor and a surface of its heat-affected zone, water jet peening (hereinafter referred to as WJP) is executed to the weld and heat-affected zone of the structural member to improve the tensile residual stress to compressive residual stress on the surface of the structural member.
The WJP is executed by immersing the structural member whose residual stress is to be improved in water and ejecting a high-pressure water jet from a nozzle disposed in the water. Cavitation bubbles contained in the water jet ejected in the water at high pressure are collapsed, so that shock waves are generated. These shock waves collide against a surface of the structural member in the water. As a consequence, the tensile residual stress on the surface of the structural member is improved to compressive residual stress. The structural member added with compressive residual stress can reduce occurrence of stress corrosion cracking (SCC).
An example of executing WJP to a core shroud in a reactor pressure vessel composing a reactor is stated in Japanese Patent No. 3530005. Furthermore, examples of executing WJP to a tubular member provided to a reactor pressure vessel are stated in Japanese Patent Laid-Open No. 7(1995)-270590, Japanese Patent Laid-Open No. 9(1997)-136261, Japanese Patent Laid-Open No. 10(1998)-10282, and Japanese Patent Laid-Open No. 2000-308927. In the methods of executing the WJP stated in Japanese Patent Laid-Open No. 7(1995)-270590, Japanese Patent Laid-Open No. 10(1998)-10282, and Japanese Patent Laid-Open No. 2000-308927 each use, a WJP apparatus provided with a nozzle for ejecting a water jet is attached to the tubular member (for example, a control rod drive mechanism housing) provided to the reactor pressure vessel. On the other hand, the method of executing the WJP stated in Japanese Patent Laid-Open No. 9(1997)-136261 uses a WJP apparatus provided with a nozzle for ejecting a water jet, which is attached to a moving apparatus.
In the methods of executing the WJP stated in Japanese Patent Laid-Open No. 7(1995)-270590 and Japanese Patent Laid-Open No. 10(1998)-10282, the WJP apparatus is attached to a control rod drive mechanism housing provided to the reactor pressure vessel. The WJP apparatus has a tubular body and a jet nozzle which is attached on the tubular body and rotatable in the circumferential direction of the tubular body. The WJP apparatus is inserted in the reactor pressure vessel and installed to a control rod drive mechanism housing by attaching the tubular body to the control rod drive mechanism housing. The WJP apparatus is immersed in cooling water filled in the reactor pressure vessel. The jet nozzle ejecting high-pressure water turns around the tubular body, and thus the high-pressure water jet is ejected from the jet nozzle to an outer surface of the control rod drive mechanism housing which is a WJP execution object and to which the WJP apparatus was attached. Cavitation bubbles contained in the ejected water jet are collapsed, as a consequence, the shock waves generate. These shock waves collide against the outer surface of the control rod drive mechanism housing, and the tensile residual stress of the outer surface is improved to compressive residual stress. Adjacent to the control rod drive mechanism housing to which the WJP apparatus is attached, there are other control rod drive mechanism housings and in-core monitor housings (hereinafter referred to as ICM housings). The water jet is also ejected from the jet nozzle to each outer surface of the other control rod drive mechanism housings and the ICM housings. As a result, compressive residual stress can be given to the outer surfaces of the other control rod drive mechanism housings and the ICM housings.
The method of executing the WJP stated in Japanese Patent Laid-Open No. 2000-308927 uses the WJP apparatus provided with an oscillatable jet nozzle for ejecting a water jet that is high-pressured water supplied, a WJP body installed with the jet nozzle, and a base plate to which the WJP body is removably mounted. When the WJP is executed to an ICM housing using this WJP apparatus, the base plate is mounted to two control rod drive mechanism housings adjacent to the ICM housing executing the WJP, and a water jet is ejected to the outer surface of the ICM housing from the oscillating jet nozzle provided to the WJP body installed to the base plate. At this time, the WJP body is located on a central axis of one of the control rod drive mechanism housings, and compressive residual stress is given to a circumferential half of the outer surface of the ICM housing being the WJP execution object by ejecting the water jet from the oscillating jet nozzle attached to the WJP body. This WJP body is moved to a position on the central axis of the other control rod drive mechanism housing and installed to the base plate. In this state, the oscillating jet nozzle ejects a water jet to the ICM housing. This allows compressive residual stress to be given to the entire periphery of the ICM housing.
According to the method of executing the WJP stated in Japanese Patent Laid-Open No. 9(1997)-136261, a WJP apparatus has a moving carriage; a lifting carriage provided to the moving carriage; a rotary drive axis having a spline structure on an outside, rotatably attached to the lifting carriage; a rotating tube surrounding the rotary drive axis, engaged with the spline structure; a lifting flange attached to a lower end portion of the rotating tube, held to the lifting carriage; and a jet nozzle attached to a rotating plate provided to an upper end portion of the rotating tube. A WJP execution object is an upper cover of a pressure vessel provided with a plurality of pipes, and is supported by a supporting member. The moving carriage moves below the upper cover, this moving carriage places a positioning head provided to the upper end portion of the rotary drive axis directly below a pipe being a WJP execution object, and the lifting carriage is lifted to insert the positioning head in the pipe. A cylindrical cover whose inner surface is in contact with an outer periphery of the rotating plate is lifted by an air cylinder installed to the lifting carriage so that the upper end of the cylindrical cover contacts the upper cover and the cylindrical cover surrounds the pipe. The lifting flange is lifted to lift the jet nozzle to make an ejection outlet of the jet nozzle face a weld between the upper cover and the pipe installed to the upper cover. Water is filled inside the cylindrical cover above the rotating plate, and the pipe installed to the upper cover is immersed into the water. The jet nozzle ejects a water jet into the water in the cylindrical cover toward the weld between the upper cover and the pipe. The rotary drive axis rotates, making the jet nozzle turn around the pipe as ejecting a water jet. Compressive residual stress is given to the surface of the weld.